1. Field of the Invention
The present invention relates to a nonaqueous secondary battery and a flame retardant for use in the battery. More particularly, the present invention relates to a highly safe nonaqueous secondary battery having excellent battery performance and to a flame retardant for use in the battery.
2. Description of the Related Art
In recent years, reduction in size and weight of electronic devices has been remarkably progressed, and with the progress, it has been demanded that secondary batteries that are used for such electronic devices should have higher energy density. An example of secondary batteries that can meet the demand is a secondary battery using a nonaqueous electrolyte solution (hereinafter, referred to as nonaqueous secondary battery) such as a lithium-ion secondary battery.
The nonaqueous electrolyte solution contains an electrolyte salt such as a lithium salt and a nonaqueous solvent. The nonaqueous solvent is desired to have high dielectric constant and high oxidation potential, and to be stable in batteries regardless of operation environment.
As such a nonaqueous solvent, aprotic solvents are used, and known examples thereof include high-permittivity solvents such as cyclic carbonates including ethylene carbonate and propylene carbonate, and cyclic carboxylate esters including γ-butyrolactone; and low-viscosity solvents such as chain carbonates including diethyl carbonate and dimethyl carbonate, and ethers including dimethoxyethane. Usually, a high-permittivity solvent and a low-viscosity solvent are used in combination.
However, the nonaqueous secondary battery may suffer from leakage of the nonaqueous electrolyte solution due to a defect involving increased internal pressure caused by breakage of the battery or any other reason. The leakage of the nonaqueous electrolyte solution may lead to short-circuit between a positive electrode and a negative electrode to cause generation of fire or burning. It may also lead to generation of heat in the nonaqueous secondary battery to cause vaporization and/or decomposition of the organic solvent-based nonaqueous solvent to produce gas. In some cases, the produced gas caught fire or caused rupture of the nonaqueous secondary battery. In order to solve the above-described problems, studies have been carried out to give flame retardancy by adding a flame retardant to the nonaqueous electrolyte solution.
As the flame retardant, phosphazene derivatives, azobis (isobutyronitrile) (AIBN), imidazole compounds have been proposed. However, these flame retardants degrade the battery performance when the amount thereof is increased for improvement of the flame retardancy. The present inventor has therefore proposed use of a cyclic nitrogen-containing compound as a flame retardant (WO 2009/157261).